1. Field of the Invention
Embodiments of the present invention relate to methods of smoothing the edges of a portable memory card and a memory card formed thereby.
2. Description of the Related Art
As the sizes of electronic devices continue to decrease, the associated semiconductor packages that operate within them are being designed with smaller form factors, lower power requirements and higher functionality. Currently, sub-micron features in semiconductor fabrication are placing higher demands on package technology including higher lead counts, reduced lead pitch, minimum footprint area and significant overall volume reduction.
While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted on a substrate. The substrate may in general be a printed circuit board, a leadframe or a tape automated bonding (TAB) tape. FIG. 1 shows a cross-section of a plurality of semiconductor packages 20 being fabricated on a substrate panel 22. Semiconductor packages 20 are typically batch processed from panel 22 for economies of scale. Each package 20 includes one or more semiconductor die 24 mounted to the substrate, and electrical bond wires 26 for electrically coupling the one or more semiconductor die to the substrate. The substrate may further include passive components 28, such as for example capacitors, resistors and inductors further enabling the operation of the package 20. Where the packages 20 are LGA (land grid array) packages such as are used in portable memory cards, contact fingers (not shown) may be formed on a surface of the substrate and coupled to the one or more semiconductor die through a lead pattern formed in the substrate. The contact fingers and lead pattern allow electrical communication between the semiconductor die in the package and a host device in which the package is used.
Once electrical connections between the die and substrate are made, the respective packages 20 on panel 22 may then typically be encapsulated in a molding compound to seal off and protect the components within the package. Once encapsulated, the respective packages 20 may be singulated from the panel 22 to form the finished packages. An example of a finished, encapsulated package is shown in prior art FIG. 2. The package 20 shown in FIG. 2 may for example be a Memory Stick Micro (M2) card developed by SanDisk Corporation of Sunnyvale, Calif. and Sony Corporation of Tokyo Japan, which commonly used in cellular telephones and other mobile devices.
Many conventional semiconductor packages, like package 20 in FIG. 2, have both straight and curvilinear edges. The package 20 in FIG. 2 includes a generally rectangular shape having sides 32 through 38 joined by rounded corners. Side 32 of the package 20 further includes a pair of notches 40 and side 36 includes a single notch 40. Many other types of memory cards similarly include a curvilinear shape having rounded edges, notches, and/or a chamfer, including the Transflash and the Secure Digital (SD) card.
Several methods are known for cutting the straight edges of a package 20 during singulation, including for example diamond saw. However, specialized cutting methods are required for cutting curvilinear shaped edges during singulation. Such specialized cutting methods include, for example, water jet cutting, laser cutting, water guided laser cutting, dry media cutting and diamond coated wire cutting. Such cutting methods are able to achieve sophisticated rectilinear and/or curvilinear shapes of the individualized integrated circuit packages. A more detailed description of methods for cutting encapsulated integrated circuits from a panel, and the shapes which may be achieved thereby, is disclosed in published U.S. Pat. No. 7,094,633, entitled “Method for Efficiently Producing Removable Peripheral Cards,” which patent is assigned to the owner of the present invention and which patent is incorporated by reference herein in its entirety.
As semiconductor packages continue to shrink, the structure within a host device for receiving and ejecting portable memory packages is becoming more delicate, and the ejection force with which smaller packages are ejected from the host device is getting smaller. Consequently, the roughness of the edges of portable memory packages is becoming a significant factor in package design, as small memory cards having rough edges may get stuck inside the host device.
Known cutting methods for cutting straight edges are effective at achieving smooth cuts. A measurement of roughness is Ra (average roughness), which is the measure of the average height of the bumps on a surface, measured for example in microns (μm). Straight edge cutting methods are typically able to achieve a roughness of Ra<1 μm. However, where a package includes curvilinear edges and is singulated by methods such as water jet or laser singulation, the edges are relatively more rough, typically about Ra=3 to 6 μm or greater. An example of a cut forming edges made by such methods is shown in prior art FIG. 3. FIG. 3 shows the packages 20 encapsulated in a mold compound 46 and including edges 50 defined by curvilinear cutting methods. As shown, the edges 50 of the package may be jagged, and may more easily get stuck within a host device.
In order to improve the surface roughness of package edges, it is known to grind package edges using for example a grinding wheel and/or polish package edges using for example a polishing rod. However, such methods do not sufficiently reduce edge roughness, and the tooling and equipment required adds cost to the fabrication process.